1. Field of the Invention
This invention relates to a focus detecting device for discriminating the state of focus adjustment from the amount of deviation between two object images in one direction formed by an optical system.
2. Description of the Prior Art
In single lens reflex cameras or the like, the so-called image deviation type focus detecting device designed to form two object images on the basis of light beams from two different portions of the pupil of the photo-taking lens and detect the focus state of the photo-taking lens from the relative positional relation between the object images is well known, for example, from U.S. Pat. No. 4,497,561.
FIG. 1 of the accompanying drawings shows an example of such image deviation type focus detecting device. In this FIG. O designates the surface of an object to be photographed, reference numeral 1 denotes a photo-taking lens in a single lens reflex camera, not shown, reference numeral 3 designates a field lens provided near the predetermined imaging plane 2 of the photo-taking lens 1 (the focal plane in the camera), reference numerals 4 and 5 denote secondary imaging lenses disposed symmetrically about the optical axis L of the photo-taking lens 1 for imaging two object images on the basis of light beams passing through different portions 1a and 1b of the pupil of the photo-taking lens 1, and reference numerals 6 and 7 designate photoelectric conversion element rows for detecting the object images imaged by the secondary imaging lenses 4 and 5. The element rows 6 and 7 are comprised, for example, of CCD (charge coupled device) or the like. The light-receiving surfaces of the element rows 6 and 7 are disposed on or forwardly or rearwardly of a plane conjugate with the predetermined imaging plane 2. Reference numeral 8 designates a mask provided near the secondary imaging lenses. The field lens 3 images the opening portions 8a and 8b of the mask 8 on the different portions 1a and 1b of the pupil of the photo-taking lens 1. In such a device, when the photo-taking lens 1 is moved leftwardly as viewed in FIG. 1 to bring about the so-called front focus state, the object images on the surface of the object to be photographed imaged on the light receiving surfaces of the respective photoelectric conversion element rows 6 and 7 by the secondary imaging lenses 4 and 5 become deviated in the directions of arrows and thus, the front focus state and the amount thereof are detected by the variation in the output of the photoelectric conversion element rows 6 and 7 corresponding to the relative deviation of said images. Also, in the case of the rear focus state, the respective images become deviated in the directions opposite to the case of the front focus state and therefore, the rear focus state and the amount thereof are detected.